The present invention relates to a method for transferring data between a plurality of nodes according to Fibre Channel Protocol. More particularly, this invention relates to a method and apparatus for controlling data frame transfer according to Fibre Channel Protocol, that is applied to a remote copy system including a plurality of storage control units separated by a great distance.
If a data center providing continuous service using applications breaks down due to any cause such as a disaster or accident, in present-day society, where computer systems are vital for any organization, it is very important to provide the continuous service by using another data center.
Therefore, a remote copy system includes a storage control unit that is located at a geographical location away from the first storage control unit, and that is designed to store a duplicate of data stored in the first storage control unit.
In this remote copy system, data transfer from a storage control unit to another storage control unit at a remote site is conducted according to Fibre Channel Protocol. In order to establish a long-distance connection between an initiator node and a target node according to Fibre Channel Protocol, it is necessary to consider the buffer-to-buffer credit (BB credit) defined by Fibre Channel layer 2 (FC-2). Specifically speaking, when data transfer is conducted by a storage control unit at a local site logging into a storage control unit at a remote site, the number of data frames that can be received at once by the storage control unit at the remote site is determined by the BB credit, which is one of the parameters at the time of login.
As an example of this kind of technique, there is a data transfer control method disclosed in JP-A-11-55314. By this method, a sender checks a frame sequence number sent to the sender and a frame sequence number indicating which processing performed by a receiver has terminated, thereby obtaining the BB credit value. Moreover, as the sender ascertains the difference between the latest frame sequence number sent to the receiver and the latest frame sequence number indicating which processing performed by the receiver has terminated, the BB credit value is increased or decreased depending on the receiver's traffic situation.
Moreover, Japanese Patent No. 3517452 discloses a data transfer control method that aims to prevent reduced data transfer efficiency due to cable transfer delay time. This publication describes a storage control unit having a buffer sending a data request to a host system during a period of time after having issued a data request corresponding to the capacity of storage means until it receives first data transferred from the host system; and a target node returning the data request to an initiator node while anticipating cable transfer delay time.
According to Fibre Channel Protocol, if the BB credit value is “0,” a sender FC port must not send the next frame to a receiver FC port until it receives R_RDY (Receiver_Ready). If the distance between the ports is small, the sender port can transfer data to the receiver port without being influenced by the delay time for propagation of the transfer data and R_RDY via the data transfer path.
On the other hand, if the data transfer path distance between these ports is great, reception of R_RDY by the sender port from the receiver port is delayed due to line delay. Consequently, if the buffer-to-buffer credit (BB credit) does not provide a sufficient number of buffers for the connection distance (or line delay), there is a problem of an increased time during which the sender FC port cannot send data frames, resulting in throughput reduction.